教材中图13-2 显示了日本的短期利率曾一度接近或是等于零。日元利率从未显示低于0点，这是一

种巧合吗，或者你能找出一些原因来解释为什么利率可能是以零为下界的吗？

Figure 13-2 shows that Japan's short-term interest rates have had periods during which they are near or equal to zero. Is the fact that the yen interest rates shown never drop below zero a coincidence， or can you think of some reason why interest rates might be bounded below by zero？